1. Field of the Invention
The present invention generally relates to a solid fuel combustion system with improved combustion and aesthetics and, more particularly, to a solid fuel combustion device with a limited travel air supply intended to, amongst other things, simplify operation and reduce emissions of air borne pollutants.
2. Background Description
In the mid 1980""s growing concern over ambient air quality caused regulators to focus on wood burning appliances as sources of significant amounts of particulate matter and other pollutants which posed a threat to human health. Hardware commonly known as xe2x80x9cwood heatersxe2x80x9d were the subject of a federal new source performance standard in 1988. This standard required the certification of all new wood heaters sold in the United States and was intended to cover only those products which were capable of burning at low air/fuel mixtures, a condition which can lead to high emissions of particulate matter (PM), carbon monoxide (CO) and other organic pollutants.
Wood burning appliances falling within the Environmental Protection Agency (EPA) definition of a xe2x80x9cwood heaterxe2x80x9d must be certified as clean burning by meeting specified emissions criteria when tested in a laboratory using standardized test methods. The standard specifically defines wood heaters based on performance characteristics, their intended use and size. Site-built masonry fireplaces, cookstoves, boilers and central heaters, and masonry heaters were exempt from this federal regulation. Fireplaces are not automatically exempt from regulation but gain exemption through application of EPA Method 28A (see 40 C.F.R. xc2xa760 (1988)) which is a standardized test method determining minimum burn rate and air-to-fuel ratio. Using this test method, any device exhibiting an average burn rate of higher than 5 kg/hr or an air-to-fuel ratio of higher than 35 to 1 is determined not to be a wood heater and is therefore exempt from federal regulation.
The EPA Method 28A is accepted as a reference method for determining specific operational characteristics of a wood burning appliance. Procedures for determining the minimum burn rate and the average air-to-fuel ratio are specified. The following discussion makes reference to specific burn rates and air-to-fuel ratios and unless otherwise specified, EPA Method 28A is the reference method for determining the specified values. Similarly, the term xe2x80x9cfull loadxe2x80x9d in the following discussions refers to the fuel load specified by EPA Method 28A and is considered representative of the largest fuel load likely to be encountered with use of the wood heater.
Numerous studies of emissions from EPA certified wood burning stoves have shown that field performance can vary widely depending on, among other things, fuel quality, mechanical degradation and operator actions. Poor or unpredictable performance, in effect, circumvents the intent of mandating EPA certified wood heaters since emissions of pollutants are not controlled as desired. While the factors of fuel quality and mechanical degradation can be remedied, operator performance is very difficult to control. Proper operation of air controls and bypass dampers is critical to ensuring proper emissions reduction in current certified stove models and the factors of installation, fuel properties, heating needs and even weather will require different operation from day to day or from household to household. With these factors in mind the actions or inactions of the operator when using the stove controls can be critical to effective stove performance.
Further and more specifically, current technology wood stoves have operator controls which if used improperly can cause poor performance. Wood stoves may include catalytic converters or tuned secondary air systems which serve to reduce emissions by enhancing combustion efficiency or combusting the pollutants within the effluent stream prior to entering the chimney or venting system. These systems require operator knowledge as the stoves and/or catalytic combustors must be sufficiently heated in order to be effective in emissions reduction. In the case of catalytic stoves, actuation of a bypass diverts the flow of combustion products through the catalytic combustor. If the bypass damper does not get actuated or the catalyst itself is not sufficiently heated and the stove is banked soon after fuel loading, the catalyst might not get lit and no emissions reductions are achieved. Similarly, there is opportunity for non-catalytic stoves to be banked too soon, even when using proper fuel, since preheating of the secondary air system is necessary to combust volatile organic materials evolved from the wood. Once the stove is banked and the air-to-fuel ratio (mass of air divided by mass of fuel) is overly reduced in these devices, flaming may cease and the wood stove might enter a smoldering phase which can last for the entirety of a fuel charge. These scenarios are supported in the field data and are considered undesirable.
Further, with the continuing concern over wood smoke, some localities, particularly in the Western region of the United States have widened the scope of their regulations to restrict or ban residential solid fuel burning devices which are not federally regulated. These include what are commonly known as fireplaces and masonry heaters. While these devices have served a need and have been popular in homes for centuries, some local regulations allow only EPA certified devices to be installed. Since masonry heaters and fireplaces are not affected facilities under federal law, no means of certifying their performance exists and the devices cannot be installed, or in some cases even used, in these localities. EPA certified wood stoves using current technology emissions control systems attempt to fill the need of fireplace customers however, the expense of added operator controls, pollution reduction equipment and, in general, heavier airtight welded construction make the cost of these devices higher than is desirable. Also, the complexity of user controls is higher than it need be for primarily decorative appliances, possibly resulting in operator error and less than desirable performance.
Fireplaces typically have little if any combustion air control and are intended primarily as decorative devices, although some models can be used as supplemental heaters as well. Inefficiencies of fireplaces result from high fuel burning rates and high air-to-fuel ratios as compared to wood stoves which are primarily intended for heating. Combustion efficiency can be relatively good due to the abundance of air and the presence of flaming; however, too much air can have a quenching effect which inhibits efficient combustion. Even if the combustion efficiency is relatively high (as indicated by low pollutants per unit mass of fuel), the uncontrolled high fuel burning rate can result in high emission rates (mass of pollutant per unit time), which is the measure of emissions of primary concern to air pollution regulators.
Currently, a great variety of wood burning systems have been described and demonstrated in the prior art. Indeed, xe2x80x9cfireplacesxe2x80x9d and xe2x80x9cwoodstovesxe2x80x9d have been in existence for hundreds of years but operationally, efficiency and pollution concerns still exist which are not adequately addressed with the current state of the art. Wood burning appliances may be classified as xe2x80x9copenxe2x80x9d or xe2x80x9cclosedxe2x80x9d combustion devices. The term xe2x80x9copenxe2x80x9d refers to un-controlled, un-regulated or fuel-lean operation as in xe2x80x9cfireplacesxe2x80x9d, while the term xe2x80x9cclosedxe2x80x9d implies controlled, regulated or fuel rich combustion as in xe2x80x9cwoodstovesxe2x80x9d. Un-regulated wood burning systems have low heating efficiency due to high flow rates of combustion or cooling air while regulated systems exhibit low combustion efficiency as a result of operating in a fuel rich range which, in turn, results in incomplete combustion of the organic components of the fuel and higher emissions.
Prior art systems have sought to improve the performance of either controlled or un-controlled devices in a wide variety of ways. In the case of fuel rich devices (wood stoves), a variety of pollution control technology intended to enhance combustion efficiency when a device is operating in a fuel rich condition have been described in the art. These include the use of complex secondary combustion air introduction systems as in U.S. Pat. No. 4,766,876 to Henry, et al. or the use of catalytic converters as in U.S. Pat. No. 4,330,503 to Allaire, et al.
Many examples of improvements to un-controlled, lean-burning combustion chambers have also been used and described for over one hundred years. While combustion efficiency is quite good relative to fuel-rich devices, low overall efficiency can result if the high sensible heat loss resulting from high air flow and relatively high fuel burning rates is not recovered. Prior art systems describe several heat recovery system which have been successful to varying degrees. These include the use of heat transfer chambers, long and tortuous flow paths and thermal mass storage, just to name a few. However, the known prior art devices are not operable at an average fuel consumption rate below 5 kg/hr when tested using accepted industry standards and in fact, in many instances, are intended to operate at much higher burn rates. This results in less than desirable efficiency for the reasons stated above. Significant overall efficiency improvement is made by reducing the combustion air flow and consequent burn rate.
In further examples, U.S. Pat. No. 4,368,722 to Lynch describes a device which, among other things, seeks to maintain a combustion zone within a fuel charge by novel introduction of controlled amounts of combustion air. The flow path and geometry of this air introduction are intended to help produce a lean combustion xe2x80x9czonexe2x80x9d whereby complete combustion can occur. However, as in all known prior art relating to fuel rich wood burning devices, the Lynch system includes an adjustable air introduction system for xe2x80x9cproviding exactly the amount of air desired for proper combustionxe2x80x9d, but the proper amount of air is not specified. In fact, the combustion air can be over-dampened since the inlet controlling damper may be closed enough to allow the system to operate in a fuel-rich, non-flaming condition. Considering the teachings of the Lynch system, a stove capable of being throttled too much is capable of non-flaming or smoldering combustion which would require a xe2x80x9cclean-upxe2x80x9d technology to handle the resultant emissions. If the clean-up technology is ineffective (do to inefficiency, degradation or improper use) no emissions reduction is achieved.
In U.S. Pat. No. 20,667 to Savage, a heat stove with air introduction is described as a xe2x80x9cself-regulatingxe2x80x9d air supply. Savage, however, is related only to the specific means and geometry of air introduction, and the range of operation is not specified.
What is needed in the art is a wood burning heater which burns standard firewood and ensures proper emissions performance independent of operator actions and minimizes or eliminates the requirements of proper control actions to achieve reduced emissions. A further need is a simply operated wood burning heater which effectively reduces emissions of pollutants while providing the decorative function of a fireplace.
It is therefore an object of the present invention to provide a combustion system with improved emissions performance in field use.
It is yet another object of the present invention to provide a combustion system having an operational range between xe2x80x9copenxe2x80x9d and xe2x80x9cclosedxe2x80x9d combustion devices where both efficiency and pollution concerns are mitigated.
A still further object of the present invention is to provide a combustion system having a minimum combustion air setting which ensures flaming, non-smoldering combustion and the assurance of emissions performance regardless of operator actions.
It is another object of the present invention to provide a combustion system which eliminates the need for xe2x80x9cclean-upxe2x80x9d.
A further object of the present invention is to provide a minimum combustion air setting which results in efficient and clean combustion regardless of the amount of fuel added to the firebox.
Still another object of the present invention is to provide a minimum air setting which provides the necessary air to maintain consistent flaming of the fuel within the firebox.
Yet another object of the invention is to provide a minimum air setting which limits the burn rate and air flow to provide a minimum burn rate of between approximately 2 kg/hr and 5 kg/hr and a minimum air-to-fuel ratio when burning the maximum fuel charge and only higher air-to-fuel ratios when burning less than a full fuel load.
Still yet another object of the present invention is to provide a much simplified combustion system which reduces emissions of pollutants over a range of heat outputs which are determined mainly by the amount of fuel added.
The present invention relates to an improvement in efficiency of a combustion chamber by reduction of air flow enabling a hotter fire chamber, a lower mass flow rate of combustion products and increased residence time of combustion products and heated air within the combustion chamber and chimney. In order to accomplish the objectives of the present invention, the combustion system of the present invention comprises a combustion chamber defined by front, rear and side walls, a ceiling and a bottom. An access door is provided for addition of fuel into the combustion chamber, and in the closed position is substantially sealed with a suitable gasket material such that a minimum of air flows between the door frame and its mounting surface during operation. The fueling door preferably incorporates transparent glass, providing for viewing of the flames, however, the fueling door may also be formed of any suitable material such as steel or cast iron or the like. A vent or flue is located in the ceiling of the combustion chamber for exhausting of the products of combustion into a suitable chimney and to the outdoors.
A substantial amount of draft induced combustion air enters the combustion chamber near the top of the fueling doors and is directed down the face of the fueling doors providing cooling. A general downward then rearward sweeping of the combustion air as it moves towards the fuel is also generated. A geometry of the air metering orifice is either fixed or of limited adjustability such that the minimum flow of combustion air required for flaming combustion of a full load of fuel is maintained at all times. The combustion air flow cannot be reduced beyond a certain point and thus smoldering and very low air/fuel ratios are avoided. Since the air metering is tuned for proper flaming combustion with the largest expected fuel load and cannot be reduced further, fuel loads smaller than the design fuel load will result in higher air/fuel ratios, thus further ensuring that sufficient combustion air is present for sustained flaming.
Furthermore, the minimum combustion air setting limits the amount of combustion air entering the combustion chamber such that too much air is not introduced resulting in inefficiency due to sensible heat loss, chemical loss (pollution), quenching of the flames, and undesirably high burn rates. Ideally, at the minimum combustion air setting the maximum burning rate of a full load of fuel is below 5 kg/hr, however, the maximum burn rate when burning a full load of fuel may be reduced to as low as 2 kg/hr depending on the size of the firebox and the desired maximum heating capacity of the appliance.
Heat output is adjustable primarily by the amount of fuel added at each fuel loading. Fuel piece size, quality and frequency of addition of fuel will also provide more or less flaming at the discretion of the operator. However, since the minimum air setting ensures that the minimum acceptable air-to-fuel ratio will be maintained, the operator can take no action resulting in an undesirable fuel rich condition.
The construction of this combustion chamber need not be air tight as with conventional wood stove designs which are intended to operate at very low burn rates (less than 1 kg/hr). Since the minimum burn rate is relatively high with the current invention, leakage into the combustion chamber may be acceptable and considered simply a portion of the combustion air flow. (i.e. air leakage into the combustion chamber is considered part of the combustion air delivery system). Therefore, an added advantage of the combustion chamber of the current invention is that it may be constructed of generally lighter gage material using common fasteners, thus reducing weight, manufacturing costs.
In one aspect of the present invention a solid fuel burning system for burning fuel includes a combustion chamber having a bottom wall, a top wall and four side walls forming an enclosure. At least one openable access door on at least one of the side walls is provided. Also provided is a fixed geometry air supply for providing a predetermined amount of combustion air to the burning fuel within the combustion chamber resulting in a maximum average burn rate of less than 5 dry kg/hr measured as a time averaged mass burn rate during a full consumption of a single fuel load consisting of any combination of cut lengths of 2xe2x80x3xc3x974xe2x80x3 or 4xe2x80x3xc3x974xe2x80x3 dimensional lumber at a dry basis moisture content of between 19 and 25%, individual fuel pieces spaced between 1xe2x80x3 and 2xe2x80x3 apart, at a loading density of between 6.3 and 7.7 wet pounds per cubic foot of combustion chamber volume and placed on a coalbed having a mass between 20% and 25% of the wet fuel load mass, the fixed geometry air supply includes gas permeable interface seams between any of the top, bottom or side walls or the at least one openable access door. A flue is connected to the combustion chamber disposed in fluid communication with the combustion chamber.
In another aspect of the present invention, an adjustable combustion air metering device for limiting the amount of combustion air entering the combustion chamber is also provided. The air flowing through gas permeable seams and in fluid continuity with the combustion chamber is complimentary to the air flow supplied by an adjustable combustion air metering device. Also, an actuating device may be provided for adjusting the adjustable combustion air metering device resulting in a minimum average burn rate of between 2 and 5 dry kg/hr measured as the time averaged mass burn rate during the full consumption of a single fuel load consisting of any combination of cut lengths of 2xe2x80x3xc3x974xe2x80x3 or 4xe2x80x3xc3x974xe2x80x3 dimensional lumber at a dry basis moisture content of between 19 and 25%, spaced evenly and at a loading density of between 6.3 and 7.7 wet pounds per cubic foot of combustion chamber volume when the combustion air metering device is adjusted to a minimum air flow position. The adjustable combustion air metering device and the gas permeable seams together may also supply a minimum time averaged flow rate of combustion air of approximately between 8 and 85 standard cubic feet per minute when the adjustable combustion air supply is adjusted to a restrictive air flow setting.
In yet another aspect of the present invention, an automatically adjustable combustion air metering device is also provided for limiting the amount of combustion air entering the combustion chamber. The automatically adjustable combustion air metering device opens to a less restrictive setting at a high fuel burn rate and closes to a more restrictive setting at lower fuel burn rates. The automatically adjustable combustion air metering device may provide an average burn rate of between 2 and 5 dry kg/hr measured as the time averaged mass burn rate during the full consumption of a single fuel load consisting of any combination of cut lengths of nominally 2xe2x80x3xc3x974xe2x80x3 or 4xe2x80x3xc3x974xe2x80x3 dimensional lumber at a dry basis moisture content of between 19 and 25%, spaced evenly and at a loading density of approximately between 6.3 and 7.7 wet pounds per cubic foot of combustion chamber volume, and may also supply a minimum time averaged flow rate of combustion air of approximately between 8 and 85 standard cubic feet per minute when automatically adjusted.
The combustion system may also include a second air metering device for providing a flow of combustion air to the combustion chamber and a sensing device for sensing the temperature near the combustion chamber. The combustion chamber may also include a linkage device for actuating the second air metering device in response to a temperature sensed by the sensing device. A holding device may also be provided for maintaining the automatically adjustable combustion air metering device in an open position and a linkage device for adjusting the automatically adjustable combustion air metering device in response to temperature changes sensed by the sensing device.